PIPELINE CATHODE PROTECTION ANALYSIS FOR IDENTIFICATION OF CORROSION AND STRESS-CORROSION FORMATION

Abstract

Based on long-term corrosion tests of 17GS steel samples of the pipeline installed in various soils of the taiga-marsh region in the central part of Western Siberia, it is shown that in the absence of cathode protection, the corrosion rate ranges from 0.11 to 0.06 mm per year, i.e. 0.085 mm per year averagely. The residual corrosion rate of samples under the cathode protection ranges from 0.01 to 0.004 mm per year or the 0.007 mm per year averagely, when the air limiting current density exceeds the oxygen limiting current density by a factor of 3-7. A further increase in the current of cathode protection leads to the intensive hydrogen release. The actual measured values of the cathode protection current density at its potentials (with the resistive component) from –1.79 V to 3.5 V exceed the oxygen limiting current density by 30–50 or more times. It is experimentally shown that at a cathode current density exceeding the density of oxygen limiting current by 54.43 times, the acceleration of hydrogen pressure reaches 0.007 MPa per day in a closed cavity at a 20 mm vacuum pressure. It is shown that the acceleration of hydrogen pressure in micro- and macro-cavities of the near-surface layer of the pipeline is not determinant. The advantageous embrittlement of cathode hydrogen is the reduction of cohesive strength of the near-surface layer of the pipe wall.